This invention relates to core/shell polymers and more particularly to core/shell polymers containing a rubber-core and an acrylic-vinylacetate terpolymer shell having excellent cold seal strength properties.
The control of latex particle morphology by seeded emulsion polymerization is well known. For example, U.S. Pat. No. 3,985,703 discloses certain acrylic core/shell polymers containing a high percentage of rubbery phase.
U.S. Pat. No. 4,375,532 discloses a multiphase core/shell polymer comprising a crosslinked elastomer core and a rigid thermoplastic polymer shell having a glass transition temperature of at least about 35.degree. C. comprising a mono alkyl maleate or fumarate, styrene and optionally a monomer selected from the group consisting of C.sub.1 to C.sub.8 alkyl acrylates and methacrylates, acrylonitrile and methacrylonitrile. The patentee states that these multiphase core/shell polymers are useful for blending with polyamides to provide toughened polyamide compositions.
U.S. Pat. No. 4,002,801 discloses a core/shell polymer comprised of a core of polyvinyl chloride and a shell on said core containing in polymerized form, one or more polymerizable monomers selected from the group consisting of .alpha.,.beta.-olefinically unsaturated carboxylic acids and their N-alkylol and N-alkoxyalkyl derivatives, acrylic and methacrylic acids and esters derived therefrom. The patentee states that by coating fabrics, textiles, papers and other substrates with these core/shell polymers, the so treated articles have improved and superior heat sealable properties, particularly in the case of dielectric heat sealing.
U.S. Pat. No. 3,661,994 discloses a multilayered rubber containing polymer comprising a polymeric core derived from a monomer selected from the group consisting of vinyl chloride, methacrylonitrile, methylmethacrylate, styrene and mixtures thereof and a rubber-type polymer coating said core, said rubber-type polymer being derived from a monomer selected from the group consisting of 1,3-butadiene and the C.sub.2 -C.sub.8 alkyl acrylates and mixtures thereof and a polymeric outer coating derived from a monomer selected from the group consisting of vinyl chloride, methacrylonitrile, methylmethacrylate, styrene and mixtures thereof. The patentee states that these rubber containing polymer particles are useful as high impact plastics and modifiers to reinforce relatively rigid types of plastics.
U.S. Pat. No. 4,385,152 discloses an acrylic polymer emulsion containing as a first polymeric component a core-shell polymer having a core formed from one or more monomers selected from styrene, vinyl acetate, vinyl toluene and acrylic or methacrylic acid esters of alcohols of 1 to 4 carbon atoms and a shell formed of the above monomers in conjunction with one or more monomers selected from acrylic acid or methacrylic acid esters and a minor amount of acrylic or methacrylic acid and as a second polymeric component a polymer composed of a major amount of one ore more monomers selected from styrene, vinylacetate, vinyl toluene and acrylic or methacrylic acid esters and a minor portion of one or more acrylic acid esters with acrylic and/or methacrylic acid. The patentee discloses that these acrylic polymer compositions can be used as coating compositions for various substrates such as wood, metals and cementitious products.
Other core/shell polymers are disclosed, for example, in U.S. Pat. Nos. 4,469,825, 3,291,768 and U.S. Pat. No. 4,468,498, as well as in an article published in Journal of Polymer Science: Polymer Chemistry Edition, Vol. 21, 2845-2861 (1983) entitled "Morphology and Grafting in Polybutylacrylate- Polystyrene Core-Shell Emulsion Polymerization" by T. I. Min, A. Klein, M. S. El-Aasser and J. W. Vanderhoff. The latter article discloses polybutylacrylate-polystyrene (PBA-PS) core-shell latex particles prepared by seeded emulsion polymerization.
In the preparation of films useful for packaging purposes, heat sealable coatings, such as acrylic coatings, are generally coated on one side of the film substrate and another heat sealable coating, such as polyvinylidene chloride (PVDC) is coated on the other side. The acrylic coated side is generally the outside of the web, the side in direct contact with the hot sealer surfaces, where good hot slip and jaw release characteristics are required. The PVDC coating is usually on the inside of the web and provides the high seal strength, good hot tack characteristics and barrier properties required for such packaging. Such a film is disclosed in U.S. Pat. No. 4,403,464. While these films have met with excellent commercial success, improved films, particularly films having improved seal strength and which employ cold seal coatings are still desirable.